From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.1 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0D2B5C43461 for ; Mon, 7 Sep 2020 04:45:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AD66A20838 for ; Mon, 7 Sep 2020 04:45:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726278AbgIGEoe (ORCPT ); Mon, 7 Sep 2020 00:44:34 -0400 Received: from pegase1.c-s.fr ([93.17.236.30]:24303 "EHLO pegase1.c-s.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725798AbgIGEod (ORCPT ); Mon, 7 Sep 2020 00:44:33 -0400 Received: from localhost (mailhub1-int [192.168.12.234]) by localhost (Postfix) with ESMTP id 4BlG093H7Vz9twhX; Mon, 7 Sep 2020 06:44:25 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id 4jSMXyYyLRd9; Mon, 7 Sep 2020 06:44:25 +0200 (CEST) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 4BlG091y3mz9twhW; Mon, 7 Sep 2020 06:44:25 +0200 (CEST) Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 4E2E88B779; Mon, 7 Sep 2020 06:44:29 +0200 (CEST) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id J1ciJfXJXz2l; Mon, 7 Sep 2020 06:44:29 +0200 (CEST) Received: from [192.168.4.90] (unknown [192.168.4.90]) by messagerie.si.c-s.fr (Postfix) with ESMTP id A193B8B75B; Mon, 7 Sep 2020 06:44:28 +0200 (CEST) Subject: Re: [PATCH] /dev/zero: also implement ->read To: David Laight , Pavel Machek Cc: Christoph Hellwig , "arnd@arndb.de" , "gregkh@linuxfoundation.org" , "linux-kernel@vger.kernel.org" References: <20200903155922.1111551-1-hch@lst.de> <55d1ecb8-4a0c-fa58-d3cf-bf6796eea7bd@csgroup.eu> <3b0b58be4b844162b73db1b108a9b995@AcuMS.aculab.com> <20200906182122.GA12295@amd> <8c353864-76a9-90bf-fa2f-f7a8231b5487@csgroup.eu> From: Christophe Leroy Message-ID: <7ece832b-e2b7-04af-f4bb-e84c909ee332@csgroup.eu> Date: Mon, 7 Sep 2020 06:44:24 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le 06/09/2020 à 22:52, David Laight a écrit : > From: Christophe Leroy >> Sent: 06 September 2020 19:36 >> Hi, >> >> Le 06/09/2020 à 20:21, Pavel Machek a écrit : >>> Hi! >>> >>>>>> Christophe reported a major speedup due to avoiding the iov_iter >>>>>> overhead, so just add this trivial function. Note that /dev/zero >>>>>> already implements both an iter and non-iter writes so this just >>>>>> makes it more symmetric. >>>>>> >>>>>> Christophe Leroy >>>>>> Signed-off-by: Christoph Hellwig >>>>> >>>>> Tested-by: Christophe Leroy >>>> >>>> Any idea what has happened to make the 'iter' version so bad? >>> >>> Exactly. Also it would be nice to note how the speedup was measured >>> and what the speedup is. >>> >> >> Was measured on an 8xx powerpc running at 132MHz with: >> >> dd if=/dev/zero of=/dev/null count=1M >> >> With the patch, dd displays a throughput of 113.5MB/s >> Without the patch it is 99.9MB/s > > That in itself isn't a problem. > What was the throughput before any of these patches? > > I just remember another thread about the same test running > a lot slower after one of the related changes. > While this speeds up read /dev/zero (which is uncommon) > if this is needed to get near the old performance then > the changes to the 'iter' code will affect real workloads. If you are talking about the tests around the set_fs series from Christoph, I identified that the degradation was linked to CONFIG_STACKPROTECTOR_STRONG being selected by default, which provides unreliable results from one patch to another, GCC doing some strange things linked to unrelated changes. With CONFIG_STACKPROTECTOR set to N, I get stable performance and no degradation with any of the patches of the set_fs series. Christophe